An electrode or wiring is created by forming a metal film in a predetermined pattern. The metal film usually is patterned using a photolithography method.
The photolithography method is useful as a method of patterning metal but has some problems. The first problem is that at least 90% of the resist liquid is wasted in a resist application process and therefore the material is wasted. The second problem is that an organic solvent, which may affect the environment adversely needs to be used in a large amount in a development process and a peeling process. The third problem is that since an expensive photomask is needed for each pattern, the manufacturing cost is high when various patterns are to be formed. In addition, there is a disadvantage in that a number of steps are required to carry out the photolithography method.
In order to solve these problems of the photolithography method, methods of forming a metal pattern using an ink-jet method or a microstamping method have been proposed in recent years. These methods are described below.
(1) Method of Forming Metal Pattern Using Ink-Jet Method
This utilizes a method of drawing images by discharging a fluid (the ink-jet method), which is used for an ink-jet printer.
A metal pattern can be formed by placing a metal paste in a predetermined position using the ink-jet method. With this method, the paste material can be used without being wasted and furthermore, the processes of developing and peeling the resist are no longer required.
In addition, there is a method of forming a resist film in a predetermined pattern by the ink-jet method, as another method that employs the ink-jet method. In this method, first, a resist liquid is applied to a metal film by the ink-jet method to form a resist film in a predetermined pattern. Thereafter, etching and resist peeling are carried out and thereby a metal pattern is formed. In this method, a resist film can be formed with a small amount of resist material. Furthermore, this method does not require the processes of exposing and developing the resist.
JP2002-299833A and JP2002-324966A disclose a method of forming a metal wiring pattern by discharging a paste containing resin and metal microparticles by the ink-jet method. JP10(1998)-204350 discloses a method of forming metal wiring by discharging a liquid containing metal microparticles with a sulfur compound adsorbed to the surfaces thereof, by the ink-jet method. JP2002-134878A discloses a method of forming a circuit pattern by discharging ink of metal microparticles by the ink-jet method. Further, JP2000-155429A discloses a method of forming a resist pattern by jetting liquid resin from a discharge head to form a desired pattern image on a substrate and then thermally curing the pattern image. JP2000-340928A discloses a method of forming a resist pattern by discharging ink that can be melted with heat by the ink-jet method, with the ink being in the molten state.
(2) Method of Forming Metal Pattern Using Microcontact Printing Method
A method of forming a resist pattern of alkanethiol using a stamp formed of silicone resin is disclosed (Appl. Phys. Lett., Vol. 63, No. 14 (1993), p 2002-2004). This method is illustrated in FIGS. 14A to 14D.
First, a stamp 201 formed of silicone resin is impregnated with an ethanol solution containing alkanethiol (hexadecanethiol in the drawings) and then is pressed onto the surface of a metal film 203 such as gold or copper (FIG. 14A). With this, a monomolecular film 205 of alkanethiol is formed on the surface of the metal film 203 (FIG. 14B). The alkanethiol is bonded to the metal film 203 through a mercapto group (—SH) and thereby the monomolecular film 205 with a thickness of 1 to 3 nm is formed. In the monomolecular film 205, since the molecules are lined up densely, it does not allow an etchant to pass therethrough and thus serves as a resist film.
Next, the metal film 203 is exposed to an etchant and thereby a metal pattern 206 is formed (FIG. 14C). Subsequently, the monomolecular film 205 is removed with, for instance, ozone or heat (FIG. 14D). It is disclosed that a gold pattern with a submicron width can be formed by this method (Synthetic Metals, Vol. 115 (2000), p 5-11).
Since the metal paste has a lower conductivity when it is not treated, the metal paste therefore needs to be baked to increase its conductivity when wiring or electrodes are to be formed with the metal paste. Accordingly, a baking step is required in the method of forming a metal pattern by discharging a metal paste by the ink-jet method. In order to obtain a conductivity equivalent to that of a metal pattern formed by the photolithography method, ideally it is necessary to bake the metal paste at a temperature around the melting point of the metal. Such a temperature, however, causes common substrates to be denatured thermally. Hence, the baking needs to be carried out at as low a temperature as possible. Particularly, when the substrate is a printed circuit board formed of resin, the baking temperature is preferably 200° C. or lower. Accordingly, the conductivity of the metal pattern formed by the ink-jet method is lower than that of one produced by the common photolithography method. The decrease in conductivity of the wiring of a circuit results in deterioration in performance of an electronic device.
The conventional method of discharging a resist liquid by the ink-jet method allows a metal pattern with a high conductivity to be formed easily, which is different from the method in which a metal paste is used. However, when a resist liquid is applied to a substrate by the ink-jet method, the resist liquid 211 that has been discharged spreads on the substrate 212 as shown in FIG. 15. Depending on the relationship between wettability of the substrate and the property of the resist liquid, the area of the resist liquid 211 that has been applied may be at least 10000 times larger than the sectional area of the discharged droplet in some cases. It therefore may be difficult to form a resist pattern with the high precision of the conventional method in some cases.
The method that employs the microcontact printing method hardly causes a pattern to spread since a monomolecular film is formed in only the part with which a stamp has been in contact. It, however, is necessary to produce a stamp for each pattern and therefore it is not suitable for cases where various kinds of products are to be produced in small amounts.